Ultrasonic transducers, which transmit and receive ultrasonic waves, enable one to view the interior of an object noninvasively. They have a wide variety of applications--a major use being ultrasonic imaging of the human body as a medical diagnostic tool.
Ultrasonic transducers utilize piezoelectric properties to convert electrical energy into mechanical energy (i.e., an electrical signal applied to the transducer generates a mechanical sound wave which is sent into the body), and vice versa, convert mechanical energy back into. electrical energy (i.e., the sound wave reflected off an internal organ is converted back to an electrical signal and sent to an imaging device). The more efficiently the transducer performs This energy conversion, the stronger the signal. Two important measures of the strength and sensitivity of the transducer material are the electromechanical coupling factor k.sub.t and dielectric constant K.
Another important factor in medical ultrasonic imaging is the stability and reproducability of the response over the operating temperature range. Medical transducers are intended to operate at room temperature (i.e., about 25.degree. C.). However, in practice the temperature in the room may actually be much lower (e.g., 15.degree. C. or lower), and the probe may heat up during normal use to a much higher temperature (e.g., 40.degree. C. and above). These temperature variations can have a substantial effect on the transducer output for materials whose electromechanical properties are temperature dependent.
Ultrasonic transducers for medical applications have been fabricated from piezoelectric materials such as lead zirconate titanate (PZT) ceramics. It is also known to fabricate transducers from a material which is highly polarizable by application of a D.C. bias voltage, the material thereby exhibiting piezoelectric properties. The material loses its polarization upon removal of the D.C. bias voltage and no longer exhibits piezoelectric properties. This property of tuning the piezoelectric effect ON or OFF by the presence or absence of a D.C. bias voltage can be observed, for example, in materials which are preferably maintained in the vicinity of their ferroelectric to paraelectric phase transition temperatures. The ferroelectric phase exhibits piezoelectric properties whereas the paraelectric phase does not. Materials having the above described D.C. bias voltage dependent properties are referred to herein as "electrostrictive" materials.
Conventional ultrasonic transducers use piezoelectric materials which exhibit a remanence in polarization after the applied D.C. bias is removed. Thus the piezoelectric activity and consequently the sensitivity of the transducer is a constant, and does not change appreciably with temperature. However, using an electrostrictive material, one can provide a transducer with controllable sensitivity which makes it suitable for such applications as a variable aperture probe, e.g., wherein an ultrasonic beam is electronically scanned in the X-direction and controlled in the Y-direction by a bias voltage. Another application is a two-dimensional array, e.g., a crossed-array electrode type 2D probe, wherein the piezo-active region can be selected in space and in time by D.C. bias field switching and a selected region is mechanically isolated by a passive polymer.
Several material families have been evaluated as potential candidates for electro-strictire transducer application. Two examples of such materials include lead-magnesium-niobate modified with lead titanate (PMN-PT), and barium-strontium-titanate (BST). The temperature dependence of polarization and dielectric constant for these different electrostrictive materials is illustrated in FIGS. 1-2.
The temperature dependent behavior of BST is illustrated in FIG. 1. This is a material having "normal" ferroelectric behavior, that is, the temperature of the dielectic maxima T.sub.max substantially coincides with disappearance of polarization T.sub.d. Thus below T.sub.max, BST exhibits a stable remanent polarization after the applied D.C. bias is removed. Such remanence in polarization defeats the goal of providing a transducer with controllable sensitivity. To operate above T.sub.max, BST requires a relatively large electric field to achieve the required polarization and is thus difficult to make into a practical device where the goal is to minimize the field applied to the transducer from the consideration of patient safety.
Another type of electrostrictive material is a "relaxor" ferroelectric, e.g., PMN--PT. In relaxor ferroelectric materials, the temperature of the dielectric maxima T.sub.max is substantially higher than the temperature corresponding to the disappearance of polarization T.sub.d. Thus, the operating temperature range for a PMN--PT type of material is between T.sub.d and T.sub.max as illustrated in FIG. 2. PMN is a relaxor material having a diffuse phase transition, which produces a broadened dielectric maxima. PT, a normal ferroelectric, forms a solid solution with the relaxor PMN; the amount of PT can be increased to increase T.sub.max. However, this does not increase the operating temperature range, but simply shifts it upwardly. A PMN--PT solid solution having approximately 90 mole % PMN and 10 mole % PT has been proposed for use as an ultrasonic transducer, having an operating temperature range of about 25.degree. C. around room temperature (i.e., a T.sub.d of 15.degree. C. and T.sub.max of 40.degree. C.). However, this may not be sufficient for use in a much cooler room, or when the device heats up.
It is an object of this invention to provide an electrostrictive transducer material having an expanded operating temperature range, and relatively small variations of sensitivity with temperature.